I think it's being downvoted because it's not a great strategy to address a distributed denial of service. In a DDOS, you have a high number of attack sources, none of them using a great amount of bandwidth on their own.
The bandwidth use isn't notable until you get pretty close to the destination. Thus, the costs are (mostly) borne by the victim. Basically it doesn't put the pain where it needs to go.
If the transaction cost can become insignificantly low, then money could flow in the direction of the victim. Each attack source could end up making a small contribution to the victim's bandwidth bill.
I'm not sure economic incentives will lead to this happening. There's also the difficulty in attributing traffic to the person who requested it ("is this a request, so we should bill the packet source, or is this a reply, so we should bill the packet destination?").
But in theory, I don't see the DDoS pattern (a high number of attack sources) making any difference to this proposal.
Besides all the complications associating with billing, wouldn't this create more incentives for those controlling the botnets to use them in a creative way to actually make money from traffic ?
I believe this is what TekMol suggested when he said that the "individual players will make it costly for the other players to send problems their way".
The bandwidth use isn't notable until you get pretty close to the destination. Thus, the costs are (mostly) borne by the victim. Basically it doesn't put the pain where it needs to go.